The differentiation of functional antigen-reactive thymus-derived lymphocytes from their precursors will be studied beginning with prothymocytes and the inductive microenvironments which enable them to develop into mature T cells. Specifically, the role of the thymic microenvironment, the signals of which might be responsible for leukemogenesis will be studied. The project will be conducted in six phases, namely: (1) studies of function of prothymocytes in immune regulation and recognition; (2) analysis of the recognition repertoire of prothymocytes; (3) the role of thymic epithelial, thymic macrophage and thymic hormone preparations as inductive influences in the differentiation and maturation of T cells; (4) analysis of the events associated with the transformation of prothymocytes; (5) interaction of normal prothymocytes and/or immature thymocytes with leukemogenic viruses or thymus-epithelium from high leukemic strain mice; and (6) analysis of virological changes in congenic mice which have been made into thymus chimeras. In order to study the goals outlined hereafter, prothymocytes will be isolated on the fluorescence-activated cell sorter (FACS IV) by staining with FIT C-rabbit antimouse brain antiserum. The main methods to be employed in answering the above questions will consist of thymic epithelial cell monolayers, absorption studies of prothymocytes on splenic adherent cell cultures and the use of specific thymus chimeras. Functional assays pertinent to the above will be performed.